1. Field of the Invention
The present invention relates to reference voltage generating circuitry, and more particularly to reference voltage generating circuitry in an integrated circuit device.
2. Description of the Related Art
In conventional reference voltage generating circuitry, a basic regulated voltage is derived from an unregulated supply, and this basic regulated voltage is then buffered to produce at an output of the circuitry a reference voltage having a desired current driving capability. The basic regulated voltage may be derived, for example, by a reverse-biased Zener diode, or a bandgap reference circuit, and the buffering may be provided by an operational amplifier.
An output impedance of such circuitry typically appears to be inductive, as the gain of the output buffering stage generally falls off with increasing frequency. As shown in FIG. 1 of the accompanying drawings, the output impedance can be modelled to a reasonable approximation as a fixed inductor. In practice, the actual inductance will not be fixed, but may vary in dependence upon such factors as output current (since the transconductance of an operational amplifier changes with current) and temperature.
Because of the essentially inductive output impedance, the output impedance Z.sub.O, as seen by load circuitry connected to the output, increases linearly with a frequency .omega. of operation of the load circuitry. This does not pose any problems in the case when the generated reference voltage is fed into "static" load circuitry, i.e. load circuitry that has no varying signals, or has signals varying only in a low frequency range where the inductor has very low impedance.
In practice, however, the load circuitry to which the reference voltage generating circuitry is connected may include elements which switch at high frequencies. For example, FIG. 2 of the accompanying drawings shows an example in which reference voltage generating circuitry 1, with an inductive output impedance Z.sub.O, is connected to load circuitry 10 which incorporates switching elements 12, such as transistors. The load circuitry in this example also includes a constant current sink element 14. A constant current I is sunk by the current sink element 14. The effect of the element 14 is to make less significant the changes in the total current drawn by the load circuitry. In this example, the switching elements 12 may be switching currents at a high frequency, for example up to 100 MHz in some applications. This inevitably produces small high-frequency spikes or glitches in the total current drawn from the reference voltage circuitry. At high frequencies the output impedance Z.sub.O, which is essentially inductive, will be high. Accordingly, any high-frequency variation in current will cause an undesirable corresponding variation in the reference voltage which is delivered from the voltage reference generating circuitry (at node A in FIG. 2).
In practice, it is desirable that the output impedance of the reference voltage generating circuitry is stable beyond the actual clock frequency applied to the switching elements themselves, as the fast switching times of the switching elements will cause higher-frequency transients to be generated.
In precision applications, for example in high-speed digital-to-analog converters (DACs) or analog-to-digital converters (ADCs) which are clocked at rates of around 100 MHz or more, the variation in reference voltage caused by high-frequency variation in the load circuitry is highly significant.
Accordingly, it is desirable to provide reference voltage generating circuitry capable of generating a reference voltage which is less susceptible to the effects of such high-frequency load variation.